1. Field of the Invention
The present invention relates to a method and apparatus for correcting signals used for a transmitter, an apparatus for compensating for distortion, an apparatus for preparing distortion compensation data, and a transmitter.
To be more specific, the present invention relates to a technique for correcting amplitude distortion or phase distortion impairing linearity and occurring in a high-frequency circuit employed in a transmitter or the like for a portable telephone so as to maintain linearity for the purpose of minimizing power leaking out of channels adjoining each transmission channel, when various kinds of communication are carried out by using a portable telephone, portable information equipment, and information terminal within a communication system such as a local area network (LAN), fiber-to-the-home (FTT), or the like or within a system within which communication is carried out between base stations and mobile stations.
2. Description of the Related Art
A high-frequency circuit employed in a transmitter or the like for portable telephones is required to save a frequency band occupied by each transmission channel or transmission power as much as possible. This makes it difficult to allow tolerance in the frequency band occupied by each transmission channel. A signal of a narrow frequency band must be transmitted over each communication channel for fear the spectrum of signals propagating over communication channels may expand very widely, due to distortion impairing linearity, and occurring in the high-frequency circuit included in the transmitter. For supplying high enough power for the high-frequency circuit including an amplifier to operate while maintaining good linearity, an attempt has been made to correct distortion characteristics of a high-frequency amplifier such as a microwave amplifier, which is highly required to ensure especially good linearity, or a characteristic thereof concerning the distortion impairing the linearity, by utilizing a feed-forward method.
Referring to FIG. 1, which will be described in "Brief Description of the Drawings", the configuration and operations of an apparatus for correcting signals in accordance with the prior art will be described so that problems underlying the apparatus for correcting signals can be easily understood.
FIG. 1 is a circuit block diagram showing an example of the apparatus for correcting signals utilizing the feed-forward method in accordance with the prior art. FIG. 1 schematically shows the circuitry, of the apparatus for correcting signals, that is connected to a high-frequency main amplifier 130, of which the distortion characteristics are the subject for correction, for the purpose of correcting the distortion characteristics concerning the distortion impairing the linearity of an output signal Sout relative to an input signal Sin.
The apparatus for correcting signals shown in FIG. 1 includes a first feed-forward amplifier 100 exhibiting substantially the same distortion characteristic as the main amplifier 130 and requiring little power consumption, and a second feed-forward amplifier 110 and third feed-forward amplifier 130 for almost perfectly canceling out error signal components generated by the first feed-forward amplifier 100.
To be more specific, in the apparatus for correcting signals shown in FIG. 1, the first feed-forward amplifier 100 intentionally produces a distortion signal according to the distortion impairing the linearity and likely to occur in the main amplitude 130. The distortion signal output from the first feed-forward amplifier 100 is attenuated properly by a first attenuating unit 310. A difference between a delayed input signal produced by delaying the input signal Sin by means of a first delay unit 210 and the distortion signal attenuated by the first attenuating unit 310 is calculated and amplified by a second feed-forward amplifier 110.
Furthermore, a signal output from the second feed-forward amplifier 110 is attenuated properly by a second attenuating unit 320. Thereafter, a difference between a delayed input signal produced by delaying the input signal Sin by means of a second delay unit and a distortion signal output from the second attenuating unit 210 is calculated in order to cancel out error signal components almost perfectly. Finally, the difference is amplified by a third feed-forward amplifier 120, and a signal output from the third feed-forward amplifier 120 is supplied to the main amplifier 130.
However, in this case, since it will not occur that the distortion characteristics of a main amplifier are identical to that of a feed-forward amplifier, when an attempt is made to almost perfectly cancel out error signal components generated by the feed-forward amplifier, the conditions for designing a circuit or the circuitry become complex.
Recently, the number of portable telephones or portable information equipment put to use is increasing drastically, and the density of frequencies of a spatial electromagnetic wave utilized by users becomes higher. The frequency band occupied by each communication channel tends to be restricted to a narrower band.
Furthermore, there is a tendency toward using a communication channel permitting a relatively wide frequency band for the purpose of realizing high-speed and large-capacity communication. The spectrum of signals propagating over communication channels therefore expands, whereby the number of intermodulation waves generated by the communication channels increases. Furthermore, the possibility that each communication channel is intercepted by adjoining channels increases.
Even in these circumstances, it is necessary to maintain low power consumption in a transmitter or the like incorporated in a portable telephone or portable information equipment, and to ensure a dynamic range of a transmission output that is wide enough to control transmission power so that reception power can be retained at a necessary minimum level all the time.
In consideration of the recent trends in portable telephone or portable information equipment, even when the method for correcting signals utilizing the feed-forward method in accordance with the prior art is adopted, the conditions for circuit designing or the circuitry required for almost perfectly canceling out error signal components generated by a feed-forward amplifier will become more and more complex. Besides, the scale of a circuit for correcting distortion characteristics will become larger. This poses a problem that power consumption required increases drastically.
Aside from the aforesaid method of correcting a distortion characteristic utilizing the feed-forward method, there are three methods of correcting a distortion characteristics of a high-frequency circuit in accordance with the prior art.
(1) Method Utilizing a Linearlizer
For improving linearity offered by a high-frequency circuit, distorted components which impair the linearity and occur in an amplifier whose distortion characteristics are the subject for correction, are canceled out by using a nonlinear device exhibiting nonlinearity, such as a diode. However, it is technologically difficult to design a circuit including such a nonlinear device in such a manner that the circuit exhibits the same characteristics as a circuit including an amplifier whose distortion characteristics are the subject for correction. Advanced technologies are therefore needed to design a circuit that does not need adjustment to enable modification of the conditions for circuit operations or to cope with the uncertainty in characteristics of a nonlinear device.
(2) Method Utilizing Cartesian Feedback
The linearity to be exhibited by an amplifier whose distortion characteristics are the subject for correction is improved by demodulating an output signal of the amplifier through quadrature demodulation, and feeding back a resultant signal for producing a baseband signal. In this case, the circuitry becomes rather complex.
(3) Method Utilizing a Two-dimensional Coefficient Table and a Predictor While Providing a Learning Ability
While a two-dimensional coefficient table is corrected by detecting distortion impairing linearity and occurring in an amplifier, a digital signal processor (DSP) forming a predictor is used to pre-distort a baseband signal. Thus, the baseband signal is corrected. This method utilizing the two-dimensional coefficient table has been disclosed in, for example, literature on a related art entitled "Improvement of Digital Mapping Predistorters for Linearizing Transmitters" written by Qiming Ren and Ingo Wolff (IEEE MTT-S Opening Forum 2, pp.0-7803-4603-6 of 2-52 in CD-ROM, June 1997). The literature on the related art has reported a case in which adjacent channel power (ACP) is reduced by 45 dB or more. In this case, since the distortion impairing the linearity and occurring in an amplifier, should be corrected with high accuracy, the storage capacity of a memory containing the two-dimensional coefficient table gets relatively large and the number of calculations performed by the DSP becomes larger. This results in increased power consumption.
In any of the aforesaid methods of correcting a distortion characteristics of a high-frequency circuit, in accordance with the prior art, the aforesaid problems remain unsolved.